Apparatus for detecting imperfections in insulating materials



Feb. 24, 1948. D. E. STEARNS APPARATUS FOR DETECTING IMPERFECTIONS ININSULATING MATERIALS Filed July 5, 1943 DICK E. STEARNS INVENT OR.

ATTORQFYS. v

Patented Feb. 24, 1948 APPARATUS FOR DETECTING IMPERFEC- TIONS ININSULATING MATERIALS Dick E. Stearns, Shreveport, La.

Application July 5, 1943, Serial No. 493,514

3 Claims. I

This invention relates to electrical testing apparatus and has for itsgeneral object the provision of an electrical testing apparatus whichwill provide an efficient means for testing insulating materials.

The particular problem to which this invention has been applied is thatof testing insulating coatings such as employed on pipe lines and thelike.

In protecting pipe lines from corrosion one methodheretofore used hasbeen that of coating the pipe with an insulating material so as toprevent the flow from the pipe of any electric current. When'a perfectinsulating coating is provided, this of course prevents any electrolysisof the pipe and damage to the pipe resulting therefrom. However, if thecoating is imperfect at a point, any electric current which may beinduced to flow from the pipe due to electrolysis at the given locationwill be concentrated at that point and will be of much greater valuethan would the electric current flowing at the given point were there nocoating whatever. Under these circumstances a hole would be produced inthe pipe much more quickly than if all other conditions were the sameand the pipe were left bare. It is therefore highly important that alldefects in the insulating coating of such pipes be detected andcorrected.

For the purpose of detecting such defects, commonly called and sometimeshereinafter referred to as holidays, there has been employed in the pasta high voltage circuit one terminal of which is grounded to the pipe andthe other terminal of which is in the form of an electrode, whichencircles the pipe and is movable along the pipe. As this electrode ismoved along the pipe a spark or arc occurs whenever a defective point isencountered where the insulation provided is insufficient to withstandthe testing voltage being applied. However, it is many times impossibleto see 2 done in many instances would make it impossible because ofother and louder noises to detect the sound of a spark.

Certain devices have heretofore been disclosed and claimed in my priorPatent No. 2,304,513, issued December 8, 1942, and in my priorco-pending application, Serial No. 446,578, filed June 11, 1942, forApparatus for detecting imperfections in insulating materials, both ofwhich devices are capable of producing with certainty Y clearly sensiblesignals whenever a moving electrode encounters any defect in aninsulating coating over which it is being moved. The present inventionconstitutes an improvement over my said prior inventions.

One object of the present invention is to provide a device of thecharacter referred to which will require fewer parts than thoseheretofore devised.

Another object is to provide such a device which will accomplish all ofthe advantageous results produced by my aforesaid earlier inventions butwhich will be less expensive and simpler both to manufacture and tooperate.

Another object of the invention is to produce a device of the typereferred to which will be so simple to operate as to be practicallyfoolproof.

More specifically an object of this invention is to provide a devicesimilar in purpose to my said prior inventions but employing only onethermionic vacuum tube and one relay and having only one externalcontrol.

Another object of this invention is to provide a device in which thenecessity for dry cell batteries for producing medium voltages will beeliminated.

Another object is to provide a device which will produce .a testingimpulse having an extremely steep wave front as it increases to itsmaximum such a spark because it may be of very slight intensity and mayoccur for only a fraction of a second. Also, it may occur on theopposite side of the pipe from the observer and it mayoccur at such apoint as to be hidden from the observer by the movable electrode.Inasmuch as such tests are necessarily conducted in many instances inbright sunlight, the visibility of any such spark will of course begreatly reduced thereby. Some reliance has been placed on the observer'sability to detect the sound produced by the spark but this isunsatisfactory because many times a spark would produce very little orno sound, and the conditions under which the testing is necessarilyvalue.

Another object is to provide a device capable of producing testingvoltages which will be steadier and more uniform in both polarity andintensity than devices for this purpose heretofore known.

Another object is to produce a structure of the type referred to whichwill be more rugged and better able to withstand the rough usage ofactual operation in the field than devices for the same general purposeheretofore known.

Another object is to provide a device of the character referred to whichwill be more dependable under all circumstances of operation thandevices heretofore known.

Another object of this invention is to provide a device which willproduce a high voltage damped storage means for producing a verysubstantial discharge spark at a holiday when such is encountered.

Other objects and advantages of this invention will become apparent fromthe following description taken in connection with the accompanyingdrawing wherein there is set forth by way of example one embodiment ofthe invention.

In the drawing the single figure illustrates diagrammatically theelectrical parts and circuits of a device constructed in accordance withthis invention.

Referring more particularly to the drawing, the numerals I and 2represent the positive and negative terminals respectively of theprimary source of D. C. power for the device, which source is preferablyin the form of a wet cell storage battery of 6 to 8 volts. It will beseen that the negative terminal 2 of this source of power is grounded tothe instrument frame which will be referred to hereinafter as theinstrument ground, it being understood that each such illustration ofinstrument ground connection be preferably connected, by appropriatewiring, at a common point on the said instrument frame. The positiveterminal I is connected through a suitable fuse 3 to one terminal of thevoltmeter 4 and the negative terminal 2 of the power source is connectedto the other pole of the voltmeter 4 so that thi voltmeter serves at alltimes to indicate the voltage of the primary source of power.

The positive terminal I is also connected to the central point of theunitary multipole control switcln5. This control switch 5 has arotatable contactor movable to contact successively a series ofterminals or contact points 5-a, 5-22, 5-c, 5-11 and 5-e. In its firstposition this movable contactor'would contact only the point 5-41. InIts second position it would contact the points 5-a and 5-4). In itsthird position it would contact the points 5-41, 6-D and 5-0, and in itsfourth position it would contact the points 5-11, 5-1), 5-c and 5-41,and so on for as many points as are carried by the controller. It willbe understood that while five such contact points are 11- lustrated inthe drawing, this control switch may incorporate as many contact pointsas may be desirable for reasons which will hereinafter appear.

When the controller 5 is in position to make contact between therotatable contactor and the point 5-a, this will serve to connect thepositive terminal I to the positive terminal of a medium voltagegenerator in the form of a commercial power pack 6. The negativeterminal of the power supply for this pack is connected to the negativeterminal 2 by being grounded to the instrument frame as illustrated. Atthe same time, the positive terminal I will be connected to the filamentof the vacuum tube 7, with. this filament or heater also being connectedto the negative terminal 2 by being grounded to the instrument ground asillustrated in the drawing. Thus, when the switch 5 is in a position tomake contact with the terminal 5-42, current will be supplied to thepower pack 6 and to the heater of the tube I. The power pack 6 may be ofany suitable commercial type producing a suificient output voltage butis preferably of the vacuum tube rectifying type of D. C. power pack forreasons hereinafter referred to. The vacuum tube I is of the tetrodetype with an indirectly heated cathode.

With the switch 5 in position to make contact through the terminal 5-a,the positive terminal I will also be connected to one terminal of agreen signal light G- indicated by the numeral 8, and to one terminal ofan amber signal light A indicated by the numeral 9.

When moved forward to its next position in which it will make contactalso with the terminal 5-2), the switch 5 will provide contact betweenthe positive terminal I and the red signal light R indicated by thenumeral I0, this signal light being also connected to the negativeterminal 2 by being grounded to the instrument ground as indicated. Thered signal light III is provided as an indication of warning to theoperator that the vibrating switch II is, as will be described, inoperative condition. This terminal 5-!) also provides connection to thevibrating switch I I which may be of any desirable commercial typehaving the proper characteristics as follows. The vibrator II has amovable double contact II-a carried on a vibrating arm and adapted tocontact at the opposite ends of its path of movement with the relativelyfixed but resiliently mounted contactors iI-b and "-0. This vibrator isactuated by means of an actuator coil II--d which serves to vibrate themovable arm carrying the contactor I I-e and cause this contactor toalternately make and break connection with the relatively fixed butresiliently mounted contactor "-4. This last mentioned movable arm iselectrically insulated from but mechanically connected to the firstmentioned vibrator arm carrying the contactor I I-a by means of somesuitable insulating connector II-g. The vibrator arm carrying thecontactor IIe is connected to the instrument ground as indicated and acondenser I2 of suitable capacity is bridged in customary fashionbetween the fixed contact point I I-j and the vibrator arm carrying thecontactor I I-e so as to reduce sparking between these contactors I I--e and I |j.

It will be seen that with current thus applied to the vibrator I I, theactuator coil II-d will cause movement of the movable contactor II-e outof contact with the fixed contactor II-f and that this will in turnde-energize the coil I I-d whereupon the arm carrying the contactorII--e will move back to its original position with the contactor iI-e incontact with the contactor iI-f thus re-energizlng the coil II-d andrepeating the process. During this time the vibra tor arm carrying thecontactor II-a will be moved first into contact with the contactor I I-band then into contact with the contactor "-0. It will be seen that thecontactor II-b is connected to the positive output terminal of the powerpack 6 and that the movable contactor I Ia is connected to one terminalof a condenser I3. The other terminal of the condenser I3 is connectedto the instrument ground as indicated and thus to the negative outputterminal of the power pack. Thus, it will be seen that when thecontactor II-a makes contact with the contactor "-12 the condenser I3will be charged with a voltage substantially equal to the output voltageof the power pack 8. Inasmuch as this When thecontactor iI--a moves intocontact with the contactor ll'-c, the condenser i3 will, it the relay2'! is closed in the manner hereinafter described, be discharged throughthe contactors 2|--a and 21-h oi the relay 21 and,

through the primary coil I5 a of the transformer l5. This discharge willbe very rapid, thus producing an immediate high voltage peak in thesecondary coil i5-b of the transformer l5. This high voltage whichincidentally is the testing voltage of the apparatus, serves to chargethe condenser I8, and upon the cessation of the discharge currentthrough the primary coil ii-a this condenser IE will tend to dischargethrough the secondary coil l5-b which with the condenser i6 forms anoscillating circuit. The net result will be an oscillatory surge ofcurrent. Each such oscillatory surge will have an initial impulse ofextremely steep wave front and of relatively great voltage intensity andafter attaining its crest voltage value it will rapidly decrease to zeropotential and, reversing polarity, reach another crest voltage value ofopposite polarity and of greatly reduced intensity. Thereupon itwillagain decrease to zero potential and, reversing polarity, reachanother crest voltage of further reduced intensity and finallycompletely die out after about three such reversals of polarity.

The number of reversals of polarity will, however, depend upon otherfactors which will hereinafter be discussed. It is also to be noted thatthere are other condenser elements hereinafter to be described which areconnected in parallel with the condenser l6 and will serve to somewhatincrease the capacity connected across the secondary winding i5-b oftransformer l5.

Referring now to the thermionic vacuum tube 1, of which this deviceemploys only one, it will be seen that the indirectly heated cathode isconnected to the negative terminal of the filament or heater and thatthe grid is connected through a fixed resistance 11 with one terminal ofthe condenser i8 and with one terminal of the fixed resistance i9.Resistor ll is provided in order to prevent any appreciable grid currentfrom flowing in the event of some inadvertent condition. The otherterminals of the condenser I it and resistance iii are grounded to theinheater and to the cathode of the tube 1. The

grid of the tube 7! is also connected through the resistance ii andthrough a further fixed resistance 20 to the fixed contactor 2ic of therelay 21 which will later be described.

Connected to the high voltage, or ungrounded terminal of the secondarycoil l5b of the transformer i5 and of the condenser I6 is one side of aspark gap 2|. The other side of this spark gap is connected to the highvoltage terminal HV designated by the numeral 22, and this terminal isin turn connected to the exploring electrode 23 which is adapted to bemoved along the outer surface of an insulating coating 24 beingdesignated by the numeral 28. This terminal 26 is connected to thoseterminals of the condenser I8 and the resistance I! which are connectedto the grid of tube 1 through resistance 11.

From this description it will be apparent that the condenser i8 is inseries with a condenser formed by the exploring electrode 23, theinsulating coating 24, and the metallic pipe 25, and further in serieswith the spark gap 21 having a negligibly small condenser efiect, andthat the aggregate condenser effect of these elements in series isconnected in parallel with the condenser i5. As long as the insulation24 does not break down or a holiday is not encountered, the charge onthe condenser l8 will be relatively very small, but in the event of suchbreakdown or boll-- day in the insulation 24, that portion of thecondenser system represented by numerals 23, 24, and 25 is destroyed asa condenser and a relatively very great charge is placed upon thecondenser IS, the result of which will presently be discussed. It willfurther be understood that, due to well known principles, the spark gap2! will provide a damping efiect upon the oscillatory surges produced bythe transformer IS in conjunction with the condenser system describedand that this damping effect will serve to greatly reduce the timeduring which each oscillatory surge will continue before dying out. Inthe event of encountering a holiday of a nature to produce asubstantially complete short circuit between the exploring electrode 23,and the surface of the metallic pipe 25, this spark gap 2| will alsoserve to prevent the condenser Hi from being discharged back through thesecondary coil l5b of the transformer l5 after having received arelatively great charge from condenser I6 upon one breakdown of thecondenser formed by the parts bearing the numerals 23, 24, and 25.

Referring now to the relay 21, this relay is of the double pole doublethrow type having two movable arms insulated from each other. One of themovable arms carries the contactor 27-11 which is adapted when the relayis closed to be in electrical contact with the fixed contactor 2'l--band when the relay is open to be in contact with the fixed contactor2ic. The other movable arm carries the contactor 2id which when therelay is closed is in contact with the fixed contactor 2'i-e and whenthe relay is open is in contact with the fixed contactor 21f. Theactuating coil of this relay is connected in the plate circuit of thetube 1 and is bridged by a condenser 28 which serves to prevent rapidvariations in the plate current from causing the relay to chatter. Alsoconnected in the plate circuit in series with the actuatin coil of therelay is a fixed resistance 29 which provides a substantial portion ofthe entire resistance in the plate circuit of this tube. The platevoltage is supplied from the positive output terminal of the power pack6. It will be seen also that the screen grid of tube '1 is connected tothe plate thereof externally of the tube. This is done in order toreduce the plate circuit resistance of tube 1 to a minimum. It is to beunderstood that tube 1 as herein used serves as a switch to operaterelay 21. This adaptation is to suddenly cut off all current from powerpack 6 through the energizing coil of relay 2! when a holiday isencountered, thus allowing relay 2! to open. It

after it will gradually allow current to increase through the energizingcoil of relay 2! back to normal value, thereby causing relay 2'! toagain close at some predetermined value of current during such build-upbetween zero and normal value. It thereby constitutes a means forproviding a delay period in the closing of the relay 2! after it hasbeen opened as described.

For the purpose of retarding sparking between the contactors Zl-a and2'|b in the event that these should tend to separate at a time while thecondenser I3 is discharging its maximum current, there is shunted acrossthese two contactors a fixed resistance 30 of a medium order, thisresistance being such as to prevent the rapid discharge of the condenserl3 when the contactors are separated, .but at the same time low enoughto permit a continued flow of current when these contactors do separate,and also thus to prevent sparking at the contactors 2la and 21-!) or awelding together thereof if they should by chance come together whencondenser H! has just been fully charged and vibrator contactor l i-ahappens to be touching vibrator contactor H-c.

It will be seen that the contactor 2l-d of the relay is connected to theinstrument ground and that the contactor 2l--e is connected to thenegative terminal of the green signal light 8 so that when the relay isclosed this green signal light 3 will be energized. Likewise, thecontactor Zl-f is connected to the negative terminal of the amber signallight 9, so that this amber signal light will be energized when therelay is opened. For the purpose of preventing sparking between thecontactors 27-11 and 2'lf, the condenser 3| and. the resistance 32 inseries with each other are shunted across between these contactors.

It will further be seen that the contactor 2l-j is connected to thenegative terminals of an audible signal device such as a bell 33, acounter 3 and any other form of electro-mechanical devices 35 such asfor example a marking device for marking a position on a pipe. Thepositive terminal of the bell 33 is connected to the contact point -c onthe switch 5, while that of the counter 3d is connected to the contactpoint 5-d and that of any other element 35 is connected to the point5-e. Thus, when the device is operated with the switch 5 moved to theposition where it contacts only the elements 5-a and 5-b, the onlysignal that will be apparent will be the amber signal light 9 which willflash when a defect in the coating 2d is encountered, together with theapproximately simultaneous extinction of green signal light 8. When theswitch is moved to the position where it makes contact with the contact.point 5c, the bell 33 will be in the signal circuit and will also beactuated when the signal circuit is energized. Likewise when the switch5 is moved to the position 5d the counter will also be actuated eachtime the signal circuit is energized and when the switch is moved to theposition 5--e, all of the different devices including the device 35 willbe actuated when the signal circuit is energized.

In the event it should become necessary or desirable to reduce thetesting voltage produced by the transformer IS, the variable resistance36 will be adjusted so as to reduce the rate at which the condenser [3may discharge through the primary coil 15-11 of the transformer l5.Also, in order to -retard any feed back from the secondary l5--b to theprimary |5--a when the condenser l8 discharges, and thus to damp theoscillations through the oscillatory circuit consisting of the condenserl6 and the secondary i5b, there is inserted into the primary circuit acheck valve 31 consisting of a conventional half wave rectifier whichmay be of the copper oxide type or any other suitable type and ofsuitable capacity. This permits relatively free flow of current in thedischarge of condenser i3 through the primary i5-a but greatly retardsany reverse flow through the rimary l'5a.

In order to disclose more completely the speciflc parts and combinationsand the characteristics thereof which have been found to be successfulin the device as disclosed, it may be stated that the device ispreferably'operated from at or 8 volt wet storage battery connected tothe terminals l and 2. The fuse 3 is a 10 ampere fuse and the voltmeterAl is an 8 volt D. C. voltmeter. A power pack '5 which has been foundsuitable is the type having a vacuum tube rectifier. This is preferredover the self-rectifying type because the output voltage polarity is thesame even though the connections to the primary power supply should heaccidentally reversed. The thermionic vacuum tube is preferably the 6V6,6V6G, or 6V6GT because such tubes, connected as shown in the drawing,provide a relatively very low plate circuit resistance at zero gridbias, and the plate current can be completely cut oil by a negative biasvoltage of the order of 25 volts applied to the central grid. All thesignal lights are of the 6-8 volt D. C. type and the vibrator is avibrating switch having a frequency of 30 cycles per second and adaptedto operate on 6-8 volts D. C.

The condenser i2 is a 1 microfarad condenser having a test voltage of600 volts D. C. and the condenser l3, a 5 microfarad condenser having atest voltage of 1000 volts D. C. The resistor 50 is a 2 megohm resistorhaving a power rating of 2 watts and the transformer 95 is of theignition coil typ The condenser it has an over-all capacity of 0.00067microfarad and an efiective test voltage of 18,000 volts but in oneinstallation was made up of three units each of a capacity of 0.002microfarad and each having a test voltage of 6000 volts, these unitsbeing placed in series.

The resistor H is a 1 megohm resistance and has a power rating of 2watts while the condenser i8 has a capacity of 0.5 microfarad with atest voltage of 1000 volts D. C., the resistor 19, a resistance of 2megohms and a power rating of 2 watts and the resistor 20 a resistanceof 1 megohm and a power rating of /2 watt. The spark gap 2| has aspacing of approximately 1 millimeter or more. The condenser 28 has acapacity of 0.5 microfarad and a test voltage of 600 volts D. C., whilethe resistor 25 has a resistance of 35,000 ohms and a power rating of 25watts. The resistor 30 has a resistance of 500 ohms and a power ratingof 25 watts.

The condenser 39 has a capacity of 0.5 microfarad and a test voltage of600 volts D. C. and the resistor 32 a resistance of 10 ohms with a powerratingof 2 watts.

The bell 33 is adapted to be operated by 6-8 volts D. C. The counter 30is adapted for operation on 6-8 volts D. C. The rheostat 36 has aresistance of ohms and a power rating of 50 watts.

It will be understood that the foregoing is by way of a specific exampleof instruments and parts found suitable for use together in connectionwith this invention but that this invention is not limited to thesespecific parts except insoi'ar as it may be limited by the appendedclaims.

In placing the instrument in operation the switch will first be turnedto provide contact with the point 5-a and if desired also with one ormore of the other points. However, until the relay 2! closes nosubstantial high voltage can be generated by the device even if thepower pack be in full operative condition and already producing itsnormal output voltage, because the points 2'|-a and 21--b will beseparated and the only possible discharge of the condenser l3 will bethat permitted by the resistor 30 which will be so slow as to preventthe generation oi. any substantial voltageby the transformer [5. Whenthe device is first turned on with the switch 5 thus placing thepositive terminal oi'the battery in contact with the point 5a, theheater oi the tube 1 will be energized, thus starting the heating of thecathode of that tube. At the same time, energy will be supplied to thepower pack 5 and this power pack, as soon as its rectifying tube becomesheated, will begin generating a noload output voltage of the order ofabout 3'75 volts if a 6 volt power source be used. Meantime, the ambersignal light 9 will be energized and will burn steadily and if theswitch is moved forward to supply energy to the bell 33 and the counter34 as well as the other mechanical device 35, all these will beenergized untilthe cathode of the tube I becomes heated to operatingtemperature. When this occurs, current will flow in the plate circuit ofthe tubeener'gizing the relay 2l.and causing the relay to close. Thisclosing of the relay will break the circuit to the signalling devices 3,33, 34 and 35 and will energize the green signal light 8 thus indicatingto the operator that the device is ready for operation. If the switch 5has previously been moved to a position in which contact is made withthe point 5-D, then the device is ready for operation except for thetact that switch 5 must be moved to make contact with the points 5-c,5-d or 5e-, as the case may be, in the event the operator wishes thesignals 33, 3B and 35 to operate. If the switch 5 has initially beenmoved only to make contact with the point 5-a, then when the greensignal lows upon operation of the relay 21, the switch 5 must be moveduntil it makes contact at least with the point 5-2) and with the otherpoints 5c, 5-d and 5-e if the operator wishes the corresponding signals33, 34 and 35 to operate. With the switch 5 making contact with thepoints 5-a and 5-22, the vibrating switch ll will be energized and willbegin to vibrate at its predetermined frequency which, in the examplegivenherein, is 30 cycles per second. When this occurs the outputvoltage of power pack 6 will be reduced by approximately 10% from itsvoltage of approximately 375 volts when the high voltage generator iswithout load. It may be noted that this frequency of 30 cycles persecond is not critical but that it has been chosen in practice becauseit is desirable that the frequency be kept as low as possible to avoidunnecessary drain on the battery providing the primary source of power,and it has been found that with the maximum speed at which these testingdevices are ordinarily moved, a frequency of 30 cycles per second forthe testing impulses is sufiicient to insure that no portion of theinsulated surface being tested is skipped between testing impulses.

With the vibrator operating, the contactor I l-a will move in onedirection into contact with the contactor ii-b whereupon the no-loadoutput voltage of the power pack Bwill be impressed upon the condenser I3 charging it to a voltage reduced below the no-load voltage byapproximately 10% as explained above. Then when the contactor il-a movesto. its opposite position and contacts the contactor li-c, the condenserl3 will be discharged through the contactors 21-1) and 21-4: of therelay 21 through check valve, or rectifier 31, through resistance unit33, and through the primary i5--a oi the transformer l5. This will berepeated at the rate 01' 30 times per second. If the vibrator should beturned on while the relay 21 is open and while the power pack 3 isgenerating its normal output voltage, the condenser is will still bedischarged through the resistance 30 but the rate of this discharge willbe greatly reduced because of the resistance 30. With the relay closedthe discharge will be so rapid as to generate an initial peak highvoltage in the secondary or the transformer l5 and it is this voltagewhich provides ever, this opposite charge will be much smaller than theinitial peak,due tothe damping effect exerted on the circuit by thespark gap 2| and the half, wave rectifier 31 as heretofore explained.These oscillations will continue until they completely die out whereuponthere will be no transient voltages whatever in the testing circuituntil the next oscillatory surge is produced by the next succeedingdischarge of the condenser l3 occurring approximately /30 of a secondlater. Considering, for example, the action taking place durin aninitial impulse of an oscillatory surge, it is to be understood that thespark gap 2! acts merely as a high voltage switch, 1. e., when thepotential across condenser l6 becomes great enough to break down theinsulation afforded by the air at gap 2! a spark or are is formedthrough the air and across the gap and the resistance across the gapelement becomes of low order, thus passing, with practically no voltagedrop thereat, charging current to the series condenser systemformed bythe exploring electrode 22, the insulating coating 24, and the pipe 25,taken together along with condenser l8. In other words until thepotential across condenser l5 becomes great enough to break down theinsulation of the air gap of spark gap 25 the resistance across the saidgap is practically infinite to the flow of current, whereas after thecritical potential has been reached and a spark or are forms, theresistance along the path formed by the spark or are itself reduces to alow value.

It may be noted that with a 6 volt power supply the charge placed uponthe condenser l3 should be of the order of 340 volts and the peakvoltage produced by the secondary of the transformer will be of theorder of 6000 volts with the exploring electrode unconnected, and withthe exploring electrode connected the peak output voltage will befurther reduced by 10% to 20% depending upon the conditions ofoperation. If a power supply of 8 volts is provided then the peak outputvoltage with the electrode disconnected should be of the order of 8000volts and with the exploring electrode connected this figure wouldlikewise be reduced by to 20%. When using an 8 volt power source,however, it may be desirable to place an additional resistance in thefilament or heater circuit of the tube l and in the filament or heatercircuit of the rectifier tube of the power pack 6 in order to prevent anundue shortening of the life of the tube.

With the exploring electrode connected and in the absence of anydischarge through the insulation 24, the maximum charge on the condenserI8 and hence the voltage applied to the grid of the tube 1 will berelatively very small and will be immediately dissipated in part by thereversal of the direction of the voltage produced in the testingcircuit. In order to compensate for the fact that the reversed voltagewill not be asgr'eat as the initial voltage, and to prevent the buildingup of an excessive charge on the condenser i8, th grid leak I9 isprovided to constantly drain oil from the condenser it any such excesscharge.

In the event of the exploring electrode 23 encountering a defect,blemish or holiday in the insulation 24, the condenser formed by thepipe 25, the insulating material 24, and the exploring electrode 23,will be in substance momentarily short circuited causing the highvoltage charge on condenser It to be divided or shared according tofundamental laws with condenser 48, thus greatly building up the amountof negative .charge on the condenser i8 and resulting thereby in a likenegative voltage being impressed on the grid of the tube 7, throughresistance ll, this, in-turn, causing the plate current of tube 5 to beimmediately extinguished. This charge will not, however, be dissipatedby the same path because even though the exploring electrode 23 becompletely short circuited through the insulation 26 against the pipe 25thus completely destroying the condenser efiect of these elements, thespark gap 2i will serve to prevent the discharge of the condenser l8 bysuch path due to the fact that the voltage shared by condenser It withcondenser it upon the occurrence of the holiday results in such agreatly reduced net voltage being placed upon condenser l8 as to make itincapable of bridging the spark gap 2|, such net shared voltage being ofthe order of 100 to 200 volts. As soon as the voltage across thecondenser i8 and on the grid of the tube 1 has been changed in anegative direction by the discharge from the initial impulse of a singleoscillatory surge, it will have attained far more than a sufficientlynegative potential for plate current cut-oil so that it will immediatelystop the fiow oi current in the plate circuit of this tube. thuspermitting the relay 2'! to open. This will immediately stop the furthergeneration of high voltage oscillatory surges by separating the con--tactors 2'|a and 21b, will cause the extinguishment of the green light 8by separation of the contactors 2'ld and 2'l-e, will cause the actuationof the signals 9, 33. 3d and by bringing together the contactors 2'!dand 2'|-,f, and will provide for the discharge of the voltage of thecondenser l8, and thus on the grid of the resistor I 9, by bringingtogether the contactors 21-11 and 21-12. Resistance l9 has been made ashigh as possible in order to dispose of as great a part as possible ofthe discharge of condenser l6 into condenser l8 in the event ofencountering a holiday, thus making for great sensitivity tube 1,through the resistor 20 as well as the and corresponding complete actionof the apparatus, as desired, even in event of a weak discharge at aholiday, while at the same time not being of so high a value as to beunable to bleed ollv the unequal charges of opposite polarity impressedon condenser 58 during normal operation in the absence of a holiday ordefect in the insulating coating. However, with such resulting highvalue of resistor E9, the time required for bleeding off the charge ofcondenser it after encountering a holiday would be excessive forpractical considerations, and "for this reason resistance at has beenadded as shown so as to assist resistance 99 in bleeding oi? the chargeof condenser it after it has been charged upon encountering a holiday.Thus the discharge of condenser l8 by resistance 69 will be aided by theresistor 20 so as to result in a. controlled desirably definite timelimit or delay before the like negative bias voltage impressed on thegrid of the tube 1 through resistance ll will be reduced sufiiciently sothat the plate current will again be permitted to flow through th s tubeand close the relay 2?. Thereby the signals 9, 33, 3d and 35 will bede-energized, the resistance 20 will be disconnected from the conditionof being in parallel to resistance It, the green signal light 8 will bere-energized, and the resumption of generation of high voltage will bepermitted. Such time limit or delay may be predetermined withinpractical limits by a proper selection of the value of the resistor 29.It is to be noted that resistor 2G in this example is V2 of the value ofresistor l9 which means that resistor 26 more than doubles the rate ofbleeding on the voltage of condenser l8 when resistor 29 is-madeoperative by the movement of relay contact 27-11 to contact 2lc. Thearrangement devised allows resistor 20 to become operativesimultaneously as the signals 9, 33, 3d, and 35 are initially energized,thus making certain that even in the event of a weak charge for anyreason being initially placed into condenser I 8 upon encountering aholiday,

the signals 9, 33, 3 3, and 35 will be actuated before resistor 20 hashad opportunity to appreciably assist resistor E9 in bleeding on thecharge of condenser l8, the object being, of course, to bring aboutactuation of the signals 9, 33, 3d,

and 35 in every case before suficient charge has been drained oil ofcondenser it so as to allow normal plate current to again flow in tube 1as hereinbefore explained and relay 2? to be reclosed thereby before ithas completely opened.

It will thus be seen that a means has been provided for carrying out allof the objects and advantages sought by this invention.

Having described my invention, I claim:

1. Apparatus for testing non-conductive material positioned on aconductive member, and locating flaws in the non-conductive material,comprising a testing circuit including a test electrode adapted to beplaced in contact with the material to be tested and an electrodeadapted to be electrically connected to the conductive member, acondenser connected in the testing circuit in series with theelectrodes, a high voltage oscillatory surge generator having its outputconnected to the testing circuit to energize the same including theelectrodes and condenser, a thermionic tube detector including saidcondenser in its grid circuit whereby the flow of current through theplate circuit of the tube is responsive to the charge on the condenser,and relay means connected to control the generator and connected to theplate circuit of the thermionic tube and responsive to' the flow ofcurrent therein to control the energizawith the electrodes, at highvoltage oscillat w of current through the plate circuit of the tube isresponsive to the charge on the condenser, ceia'y means connected to comtrol the generator and connected to the plate cir cult of the thermionictube and responsive to the flow of current therein to control theenergization of the generator whereby in operation the generator isnormally energized, and is efiectively de-energizcd when the charge onthe condenser increases as when the test electrode encounters a flow inthe material tested, and means for bleeding the increased charge fromsaid condenser whereby the generator will be energized again.

3. Apparatus for testing non-conductive material positioned on aconductive member, and loeating flaws in the non-conductive material,comprising a testing circuit including a test electrode adapted to beplaced in contact with the material to be tested and an electrodeadapted to be electrically connected to the conductive member, acondenser connected in the testing circuit in series with theelectrodes, a high voltage oscillatory surge cexierator having itsoutput connected to the testing circuit to energize the same includingthe electrodes and condenser, a thermionic tube detector including; saidcondenser in its grid circult whereby the how of current through theplate circuit of the tube is responsive to the charge on he condenser,relay means connected to congenerator and connected to the plate circuitof the t e ionic tube and responsive to the flow of c e il therein tocontrol the energizatiou of the ge crater whereby in operation thegenerate; is normally energized, and is efiec "Lively de energizeci whenthe charge on the concieilser increases as the test electrode ehcouhtersa flaw in the material tested, and meats actuated by the flow of cutsehtthrough theplate circuit the thermionic tube for hleeding said chargefrom. the condenser at a predetermined rate when said charge exceeds apredetermined value, whereby the interruption of the generator willcontinue for a finite period of time.

DECK STEARNS.

REFERENGES (SHED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS 2,306,529 Davis Dec. 29, 1942

